Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps and backlights. However, OLED devices typically have a highly reflective back electrode to enhance the output of emitted light through one side of the OLED device. This highly reflective back electrode also reflects ambient light, thereby reducing the ambient contrast ratio of the OLED device. As is known in the prior art, circular polarizers can greatly reduce the reflected ambient light, but such circular polarizers are expensive.
Significant portions of emitted light may also be trapped in OLED devices. Scattering layers may be employed to improve the light emission of OLED devices, but may inhibit the effectiveness of circular polarizers, and do not selectively absorb ambient light. Chou (WO 02/37580, filed 9 Mar. 2001 in the name of 3M Innovative Properties Company) and Liu et al. (U.S. Publication No. 2001/0026124, filed 23 Mar. 2001), e.g., taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has optical index that matches these layers. Light emitted from the OLED device at higher than critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The light-emitting efficiency of the OLED device is thereby improved, but the ambient contrast is not significantly changed.
One prior-art approach to improving OLED device contrast is to employ a black matrix in all non-emitting areas of an OLED device, as described, for example in U.S. Pat. No. 6,936,960, issued 30 Aug. 2005 to Eastman Kodak Company, by Cok, entitled “OLED Displays Having Improved Contrast”. The black matrix absorbs the fraction of ambient light incident upon the device between the light-emitting areas, without absorbing emitted light, thereby improving the contrast of the OLED. Generally, it is preferred to maximize the light-emitting area in an OLED device to reduce the current density in the light-emitting materials and extend the lifetime of the OLED. However, this reduces the amount of area available for a black matrix, thereby increasing the amount of ambient light reflected from the OLED back electrode and reducing the contrast of a top-emitting OLED device.
Other techniques for reducing ambient light reflection include the use of contrast enhancing films. For example, WO 2005/059636, filed 24 Nov. 2004 in the name of Konin-Klijke Philips Electronics N.V., by Kurt, describes a film having a plurality of waveguides separated by interstitial areas formed as narrowing recesses coated with a reflective layer. However, such a design requires small, high-precision features that are expensive to manufacture. Moreover, any imperfection in the reflective layer reduces the absorption of the ambient light and ambient contrast ratio of the device.
There is a need; therefore, for an improved organic light-emitting diode device structure that increases the ambient contrast ratio of an OLED device.